Vaccinations minimize the risk of infectious disease, but cannot prevent disease in all circumstances. Equine vaccinations can be divided into two groups: Core Vaccinations, which all horses should receive, and Risk-Based Vaccinations, which depend on a horse’s use, living conditions, age and other risks of exposure to diseases.

[Download a Vaccination Form here. (pdf format)]

Core Vaccinations

The American Veterinary Medical Association defines core vaccinations as those ”that protect from diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety.” These vaccines protect against diseases that have a high mortality (death) rate if a horse becomes infected. They are all diseases that are acquired from the environment.


All horses are at risk of tetanus, a usually fatal disease. The causative organism, Clostridium tetani, is present in the feces of horses and other animals, including humans. It is ubiquitous and abundant in the soil, and will survive there for many years, resulting in an ever-present risk of exposure. While not a contagious disease, tetanus is acquired via infection of a wound, especially a puncture wound, or through other exposed tissue.

Eastern/Western Encephalomyelitis

These diseases cause inflammation of the brain and spinal cord. If acquired, Eastern Encephalomyelitis (EEE) is 90% fatal, Western Encephalomyelitis (WEE) is about 50% fatal. Transmission of these diseases is by mosquitoes from infected birds and rodents. Humans are also susceptible to these diseases. EEE is primarily seen in the eastern and southeastern United States. While WEE is seen primarily in the western and mid-western US, WEE variants have sporadically caused outbreaks in the northeastern and southeastern states.


While the incidence of rabies in horses is low, the disease is always fatal, and has considerable public health significance. Equine exposure usually occurs from the bite of a rabid animal (primarily wildlife), usually on the muzzle (since horses are very curious) or legs. The virus travels via nerves to the brain, where it causes inflammation and death. The incubation period in horses can be quite long, and symptoms near the end can mimic colic. Transmission to humans can occur via saliva of infected animals contacting open wounds.

West Nile Virus

West Nile Virus (WNV) first entered the US in New Jersey in 1999. It rapidly spread up and down the east coast, and then nationally with surprising speed. WNV is the leading cause of arbovirus encephalitis in horses and humans in the US. The virus is transmitted from an avian reservoir to horses, humans and several other species of mammals. The virus is not transmitted directly from horses to humans, or humans to horses. The fatality rate for infected horses is approximately 33%, with 40 % of surviving horses showing residual effects.

Risk-Based Vaccinations

Equine Influenza

Influenza is one of the most common respiratory diseases of horses. It is a highly contagious viral disease and spreads rapidly thru groups of horses via aerosolized droplets dispersed by coughing. The disease occurs sporadically, introduced by an infected horse, so quarantining all new additions for 14 days is a major factor in preventing influenza. The severity of clinical signs depends on the horse’s degree of immunity. All horses should be vaccinated against influenza unless they live in a closed and isolated facility, with no equine-related visitors.

Equine Herpes Virus (EHV-1, EHV-4) 

Both EHV-1 & EHV-4 are infectious diseases of the respiratory tract, causing disease that can range form sub-clinical to severe. It is especially common when young horses from different places are commingled. EHV-1 also causes abortion in late pregnancy mares, and paralytic neurologic disease. It is spread by nasal secretions, either directly or carried on people and equipment, and by aborted fetuses and fluids in the case of EHV-1. Vaccination can protect against the respiratory and abortion forms, but not the neurologic form.

A genetic variant of EHV-4 has recently been seen. This form can cause severe neurologic disease and frequent deaths. Vaccination against the respiratory form is recommended for young horses, and for horses showing, or living with horses that travel and compete.

Lyme Disease

While there is no “approved” Lyme vaccine for horses, anecdotal evidence suggests that the dog vaccine, while “extra-label”, is very effective in horses.We will therefore be offering this vaccine to our equine patients. The recommendation for initial vaccination is one dose on day one, one dose 3 weeks later and a third dose at 3 months. Vaccination is then recommended every 6 months. We will be offering reduced-cost Lyme vaccination clinic days starting in February; contact us for details.
More on Lyme Disease

Potomac Horse Fever

Recently re-named Equine Monocytic Erlichiosis, this is a sporadic disease seen primarily in the late spring to early fall. It has been associated with high populations of Mayflies. Signs may be mild to severe, and include fever, lethargy, laminitis and diarrhea. Vaccination is only recommended for those horses traveling to areas of high incidence. The vaccine has become less protective against the current strain.


Strangles is a highly contagious respiratory disease characterized by moderate to high fever, swollen lymph nodes, and copious, thick nasal discharge. It is transmitted by direct contact with an infected horse, contact with contaminated feeding or watering equipment, grooming tools, tack or people. It can survive in the environment for a variable period of time. Vaccination is recommended for horses living where strangles is a persistent problem, or for horses at high risk of exposure.